This invention relates generally to techniques used in dipmeter well logging, and more particularly to new techniques for automatically processing dip and azimuth measurements to produce more useful three-dimensional representations of subsurface formations.
A common method of measuring the dip angle and direction or azimuth of subsurface formations employs a dipmeter tool passed through a borehole drilled into the subsurface formations. This tool may apply any of numerous means to obtain signals representative of variations of a particular formation characteristic, such as its resistivity. One such tool is described in the paper: "The High Resolution Dipmeter Tool", by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log Analyst.
Dip and azimuth measurements representing the inclination of a formation feature may be determined from dipmeter signals containing information representing the intersection of such a feature at three or more radially spaced points on the borehole surface. The displacement between two points intersecting a common feature may be determined by correlating corresponding pairs of the dipmeter signals each representing the common feature in the correlated portion of the signals. Two displacements between three different points determine the position of a plane. The position of the plane is conveniently expressed by its dip .THETA., an angle measured from a horizontal plane, and its azimuth .PHI., an angle measured from a reference direction (usually true North).
Typically, the dipmeter signals are recorded on computer compatible magnetic tape at the well site for later processing. The recorded signals are processed using any of several techniques. Manual, semi-automatic and fully automatic processing may be used with the automatic processing being performed with either analog or digital computers. When digital computers are used, a computer program is also required.
A computer program to perform the digital processing operations is described in a paper, "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tape" by J. H. Moran, et al. and published in the July, 1962 issue of the Journal of Petroleum Technology. An additional computer program is described in the paper, "Computer Methods of Diplog Correlation" by L. G. Schoonover, et al., pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Further, programs to process digitally-taped dipmeter data may be obtained from digital computer manufacturers, such as IBM.
Results from digital processing are normally presented in tabular listings of dip and azimuth measurements versus borehole depth. However, when large numbers of such measurements result, as from recent high resolution dipmeter techniques, such listings are usually augmented by graphic presentations to allow easier interpretations of these measurements.
The interpretation of dip and azimuth measurements is directed toward two general objectives, the determination of either stratigraphic or structural features. Stratigraphic features of interest are typically depositional in nature, such as the type of deposition, internal cross bedding, direction of transport, thickening, etc., of the particular stratigraphic bed. Structural features of interest are the type of structure, the nature of its controlling feature such as a fault, plane, fold axis, its symmetry, etc., and the orientation of this feature, i.e., its dip, azimuth, depth and distance from the borehole, for example.
Graphic displays or dip and azimuth representations vary with the interpretation objective. Accordingly, relationships between corresponding dip and azimuth measurements with depth and distance are considered in different manners.
For stratigraphic purposes, the trends of adjacent dip measurements with depth are usually used to classify the measurements into groups for analytical purposes. For example, dips representing a trend of rapidly increasing dip with depth will be considered separately from dips representing a trend of rapidly decreasing dip with depth. A trend of increasing dips with depth in a given direction is usually attributed to crossbedding characteristic of channel fill deposition and is often referred to as "red" patterns in this art. Similarly, a trend of decreasing dip with depth in a given direction is usually attributed to current bedding, as found in thin sedimentary structures, and is often referred to as "blue" patterns.
In the stratigraphic analysis, it is important that the azimuth of these dips must remain substantially constant and thereby represent the general direction of sediment transport or perhaps the probable direction of down dip thickening. Also, dipmeter results are combined in a given analysis from relatively short depth intervals corresponding to a given depositional or stratigraphic unit.
Thus graphic displays used for stratigraphic analysis often ignore the actual depths once the above dip versus depth trend qualifies a group of measurements. Further, since in many cases the actual dip angle is not important and only the dip azimuth is significant, the dip angle may be completely ignored in the graphic display. Such displays are designed to statistically determine the azimuth corresponding to a primary and perhaps a secondary direction of transport or deposition.
Graphic displays used in stratigraphic analysis are typically the azimuth frequency plot (no dip or depth representation) and the Schmidt net and the Stereonet (azimuth versus dip but still no depth representation). These nets and several variations thereof have certain statistical characteristics in that they may be used to enhance either low or high dip measurement point groupings. Note that each given dip and azimuth measurement is combined and represented by a point in these nets. A description of some of these displays and their application is given in the paper "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published September 1968 in the American Association of Petroleum Geologists Bulletin.
Stratigraphic and structural analyses distinguish themselves in the type of information needed. In stratigraphic analysis, the dipmeter signals hopefully represent bedding planes within the boundaries of a given stratigraphic unit. These bedding planes have little, if any, regional extent. In structural analysis, a deliberate attempt may be made to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata. Here these boundaries have large regional extensions which are of interest in mapping.
Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to obtain stratigraphic information while long lengths (10 to 20 or 30 feet) of signals are often correlated to obtain structural information. While use of long correlation lengths to obtain structural dip has been standard practice for some time, there are certain disadvantages associated with this practice. One is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computations must be made using a short length to obtain stratigraphic information. Another is that most long correlation length techniques may be influenced by frequently occurring stratigraphic features having a common dip and direction, even though each such feature is less pronounced than the structural features. Thus, the use of long correlation lengths does not assure obtaining accurate structural dip information. Yet another disadvantage is that current correlation techniques tend to ignore possibly objectionable effects of rotation of the recording tool within the long correlation interval.
Accordingly, it is therefore an object of the present invention to provide a technique of determining more accurate structural information without necessitating long correlation lengths.
In some cases dip and azimuth measurements derived from short correlation lengths are averaged over a given interval to obtain a representative dip for the interval which, under some circumstances, might be regarded as structural dip. Even though intervals for such averaging may be selected to contain measurements more-or-less conforming to an expected structural dip and azimuth in order to improve the result, this selection process requires considerable manual study. This adds both time delays and expense to the process.
It is, therefore, an additional object of the present invention to provide an automatic technique to accurately determine structural information without the necessity to pre-select the dip and azimuth intervals.
Structural dip measurements are often associated with patterns, as are the stratigraphic dips. For example, long intervals of relatively constant dip and azimuth are known as "green" patterns in this art. However, when individual bed boundaries, which may have been substantially parallel when deposited, are subsequently folded, faulted, rotated or warped in some manner, the dip angles will change from bed to bed. If, in addition, the now non-parallel boundaries are tilted, the azimuth measurements of the boundaries will also change from bed to bed. Such structural distortion tends to complicate both the stratigraphic and structural pattern recognition methods.
Special techniques are required in using graphic displays for dip pattern recognition where structural changes impose changes in the expected dip and/or azimuth trends. In some cases, the structural change can be regarded simply as a general tilting of a stratigraphic section, for example. This change can be represented by a known tilt angle and direction. In such cases, it is possible to remove this tilt from the dip and azimuth measurements throughout the entire affected interval before using these measurements in the previously mentioned graphic displays.
However, when the structural change cannot be represented by a single tilt angle and direction; i.e., represented by a change in a single dimension, or this change is undefined, there may be problems in defining and representing the structural change with depth and in the removal of the varying degree of structural change from each stratigraphic unit.
Therefore, a further object of the present invention is to provide a technique of determining representations of structural features which reflect changes in more than one dimension, for example, where the tilt angle and/or direction changes with depth.
Structural changes, varying with both depth and direction, also complicate graphic representations of dip and/or azimuth with depth. For instance, the prior art practice of representing apparent dips in a given direction versus depth--sometimes known as STICK plots because of their appearance--give an inaccurate indication when changes in both dip and direction take place with depth. For example, a change in dip may be indicated when only a change in the direction of the dip takes place. In this sense, such plots are only two-dimensional representations in that they only represent dip in one plane versus depth.
The above limitation of displaying dips versus depth in only one given direction is somewhat overcome in a graphic display known as a FAST plot. Here the dips are traced on a cylinder with its axis scaled versus depth, such that the apparent dips are viewable in whatever given direction the viewer positions the cylinder. However, the viewer can only contemplate one direction at a time.
Accordingly, it is an object of the present invention to provide a technique for producing graphic representations of changes in dip and azimuth measurements with depth in more than one direction or plane at a time.
In addition to changes of dip and azimuth with depth, it is useful to study changes with distance from the borehole. Toward this end, the geological cross-section or the contour map is frequently employed. Either may be regarded as a map. The cross-section is a map of traces representing the intersection of formation surface with a substantially vertical plane aligned in a given direction, thus representing apparent dip of the surface with depth and distance measured in the plane of a given horizon. The contour map is a map of traces representing the intersection of a formation surface with a horizontal plane, thus representing changes in depth with direction and distance; i.e., with various N-S and E-W coordinates. Since three dimensions are represented in such maps, we define such representations as three-dimensional representations, even though they are usually presented as two-dimensional plots.
The cross-section is a useful graphic representation of dips of formations with depth in a given direction over a substantial distance away from a given borehole. It may also be useful to relate formations found in two or more boreholes included in substantially the same vertical plane. A STICK plot constructed in this plane from dips in one or more of the boreholes facilitates the drawing of such a cross-section. When the formations are planar, the dips represented by these plots determine the slope of the lines corresponding to formations extending away from the borehole representations.
However, when the dip of the same formation is shown to vary from borehole to borehole, as shown by different dips on the STICK plots or as confirmed by correlation of a given formation from borehole to borehole, the formations can no longer be considered as planes. The construction of the cross-sections where dips vary with distance; i.e., non-planar surfaces, thus becomes much more complex, as straight line extrapolations of the dip away from the borehole are no longer accurate.
It is therefore an additional object of the present invention to provide a technique for producing graphic representations of changes in dip with distance away from a borehole in a given direction or azimuth.
Maps are standard tools in structural geology. They may represent any three given parameters or dimensions. Two dimensions are usually geographical coordinates. The third dimension may vary with the type of map. For example, the common topographical map represents the three dimensions of the actual earth's surface. The elevation of the surface is the third dimension in this case.
The depth of a given formation surface or stratigraphic horizon measured from an arbitrary reference plane, such as sea level, is frequently used as the third parameter of dimension in structural mapping. Where this horizon reflects structural information, such a map may properly be regarded as a structure contour map. Here, equal-depth or depth contour lines parallel the dip strike lines; i.e., .+-.90.degree. to the direction of the dip angle. The distance between contour lines varies according to the dip angle. Shallow dips are indicated by widely spaced lines and steep dips by closely spaced lines.
Dip measurements play an important part in creating structure contour maps. A representative dip and azimuth measurement is derived for the horizon being contoured. This dip angle is projected from the borehole position in the direction indicated by the dip azimuth. Contour lines are then positioned along this line at a spacing consistent with the dip angle.
Contour lines are rarely straight. Straight lines only result when the surface being mapped is a simple tilted plane. Nature is rarely so simple. Even horizons which may have been originally planar are subjected to structural changes such as folding, faulting and drape. These changes are of interest in hydrocarbon exploration in that they form potential traps for the hydrocarbons. Thus, determining the direction and degree of curvature of contour lines is fundamental to structural mapping.
Prior art methods of contouring frequently employ additional geophysical information such as seismic data obtained from surface exploration methods. Information from other boreholes are also used, such as the map locations, depth of intersection and dip of the surface being mapped. Contour lines are arbitrarily curved to match any difference in dips at each borehole.
There have been prior art attempts to extract guidance in contouring a given horizon from dip and azimuth measurements in a single borehole taken from above and below the horizon of interest. For example, the document, "Fundamentals of Dipmeter Interpretation" published in 1970 by Schlumberger Ltd., New York, N.Y. suggests there is a way to translate such information into a structural contour map when varying dip with a constant azimuth and varying dip with a varying azimuth are observed (see pages 84-87). A concept of "wall image on the structural horizon" is mentioned.
The above concept assumes all structural horizons are isometric reproductions of each other. An axial plane must exist which defines the major features of the structure. This plane must not parallel the borehole if dipmeter measurements are to be useful in defining the structure. The well image is visualized by making the shortest possible translation of each horizon parallel to the axial plane which will, in effect, reduce the thickness of each horizon to zero. This gives a well image which is essentially perpendicular to the strike of the axial plane and passing through the borehole.
In practice what is actually done is to draw a line perpendicular to the strike of the major feature; i.e., its axial plane. Points are distributed along this line corresponding to each dip in the order of their depths, their spacing being wide for shallow dips and close for steep dips. Contour lines are curved to run perpendicular to the dip azimuth at each dip point.
While the above practice is helpful, the necessity to rely on intuition or a pictorial concept in determining the distribution of the dips and the positions of the contour lines makes the method lack in accuracy and consistency. In addition, in order to be of any meaning at all, it requires a skilled interpreter to make this determination. Further, the determination indicates the nature of the structure in only a small portion of the area around the borehole; i.e., in a single plane normal to the axial plane and passing through the borehole.
Another object of the present invention, therefore, is to provide a technique of automatically producing three-dimensional representations of formation features from dip and azimuth measurements which accurately represent variation in structural features with both distance and direction from a borehole.
In accordance with these and other objects of the present invention, apparatus and methods are provided for automatically producing with a machine three-dimensional representations of formation features from measurements indicative of dip and azimuth in a borehole beneath the earth's surface. In one form of the invention, computations of apparent dip representations in two different given azimuths are made from dip and azimuth measurements of formation features at various depths. A three-dimensional representation of these apparent dip representations is produced.
The apparent dip representations in the three-dimensional representations may be in the form of traces corresponding to the intersections of the formation features with planes aligned in the directions of the given azimuths.
The apparent dip traces for each plane may be positioned on their respective sides of a line representing the intersection of the two planes. A representation of the borehole may be added to the three-dimensional representation when the borehole deviates substantially from vertical. One or both of the planes aligned in the direction of the azimuths may also be deviated to include the borehole.
In a core complex form of the invention, a structural model is determined which represents formation dip and azimuth at different depths and thereby the dip and azimuth at different distances and directions from the borehole. This model is then utilized to compute the orientation (depth, dip and azimuth) of the formation feature with distances from the boreholes, which allows extending a dip and azimuth of a formation feature at the borehole away from the borehole. In the case of traces corresponding to the intersection of the formation feature with a plane, the traces are modified in accordance with the model predicted change in apparent dip in the direction of the plane.
The determined structural model is also utilized in producing other forms of the invention, such as a contour map of a given formation feature. In this case, the dip and azimuth of a formation feature at the borehole is extended both in distance and in direction from the borehole.
The determination of the structural model is made from the dip and azimuth of formation on features at several depths. The dip and azimuth is converted to a vector positioned at the borehole intersection of a surface in the structural model and therefore, defines one point on one surface in a three-dimensional representation of a model composed of a family of similar surfaces. Tests comparing the vectors further define types of surfaces such as tilted, intersecting, co-planar, faulted, folded, etc., and the direction of such structural features.
Equations corresponding to the type of surface are matched to the dip, azimuths and depths. These equations are then utilized to compute the characterization of the surface at any position on a selected surface relative to its borehole intersection. The characterization of the selected surface may be presented in three-dimensional representations in the form of trace intersections in one or more planes. When the plane is substantially vertical, the traces appear as dip representatives . When the plane is horizontal, the traces appear as contour lines.
In some cases, the representative structural information used to determine the structural model is provided by information from other sources such as nearby boreholes. An additional form of the invention provides representative structural information from measurements indicative of dip and azmuth in the instant borehole. A three-dimensional representation of the distribution of dip and azimuth is produced and a statistical analysis is performed to determine the dominant mode of the distribution and thereby the structural dip and the corresponding depth interval.